I have a variety of Solarforce lights that accept P60 LED modules. (For those unfamiliar, the Solarforce (and others) are a knock-off of the Surefire 6P.) These lights are very modular and lego-able.

It would take about 10 seconds to install a drop-in module that uses a RED CREE X-PE2, driven at 1.7A, that makes 400-450 lumens (about 6W).

This would be brighter than any taillight I've seen for sale on the market, and it would cost about $30 (I already have the host flashlight and batteries).

The issue I'm dealing with at the moment, is how to mount it?

The bike I'm using doesn't have racks or panniers. I tried using a twofish lockblock to strap it to the seatpost, but it is too thick and my thigh hits it with every pedal stroke. Mounting to the chainstay results in the heel of my foot hitting the light. It seems like the only way to attach it to this bike may be to mount it directly behind the seatpost, but I don't know of any off-the-shelf mount that can do that.

Has anyone ever used a flashlight as a taillight? How did you mount it?

That mount that is shown in the second picture of the third post is very similar to what I have. It was for my primary front light, but I've been wanting to get one of those high powered red flashlights.... anyway, just to figure out how to mount a flashlight before ordering the red one, I started experimenting. I got that mount on the seat post- the light is not pointed straight back, but with a ~200 lumen light, I'm not sure you want that pointing straight into the drivers behind you anyway.

I only notice the body of the light rubbing against me if I'm coasting with my left leg/crank in the down position and bringing my thigh against the top tube. When I pedaling, I don't feel it rubbing against me- and I was wearing some rather baggy cargo shorts for that brief test run.*

*This was done during the daylight, with the light off, so I was in no danger of breaking any laws.

A plastic pipe that I have drilled a couple of holes in, some zip ties and anti-slip tape:

RJ, very nice idea with the plastic tube. I too wanted to mount a 501-B torch under the seat. My solution was to mount the torch directly to one of the seat rails. I was able to keep it in place by using a rather unique velcro strap that came with one of my old Blackburn seat bags. My seat has a center slot in it and using this to thread the velcro through somehow it works.

The end of the torch comes out very near my leg. Usually my leg will not hit it and I like that I can easily turn it off/on while on the fly.

I already own a very bright rear set-up ( Xeccon Geinea 1 rear ) that uses a red Cree XP-E emitter. The reason I wanted to mount the torch is so I could use something for "Daytime visibility"without draining the battery on the Geinea. Currently I am using one of the Nailbender ( Customlites ) drop-ins. My choice for Daytime was Amber coupled with their unique multi-flash-menu driver. As someone mentioned it is very narrow but for *my purpose ( *getting drivers attention at distance in the daytime ) I think it works pretty good for the money spent. I could wish for more output but then you start talking about multi-emitter set-ups ( like the DiNottte lamps ) and larger batteries which of course means lugging around more bulk and spending WAY more money.

Eventually I will replace the two-cell battery on the Geinea with a larger capacity two cell or just link the rear light to a larger battery to power both front and rear. Doing this will allow me to use the Geinea during the day more without worrying about losing run time for night use. At least with the Geinea you have the remote switch which allows you to operate the lamp ( on the fly ) only when you think you need it.

It would take about 10 seconds to install a drop-in module that uses a RED CREE X-PE2, driven at 1.7A, that makes 400-450 lumens (about 6W).

This would be brighter than any taillight I've seen for sale on the market, and it would cost about $30 (I already have the host flashlight and batteries).

Not sure about a 400+ lumen output with just one XP-E2. Normal driving current for these is listed around 700ma. Yes you can over drive the emitter but that is only going to give you so much. Don't believe output curves are going to be infinitely linear. At some point the curve will taper off as the emitter reaches it's heat limit. You might be able to run at 1.5A and that might get you something like 230-240 lumen...hard to say for sure. Surly you can also expect the output to drop significantly once the emitter begins to over heat ( which it will do when over driven )

Not sure about a 400+ lumen output with just one XP-E2. Normal driving current for these is listed around 700ma. Yes you can over drive the emitter but that is only going to give you so much. Don't believe output curves are going to be infinitely linear. At some point the curve will taper off as the emitter reaches it's heat limit. You might be able to run at 1.5A and that might get you something like 230-240 lumen...hard to say for sure. Surly you can also expect the output to drop significantly once the emitter begins to over heat ( which it will do when over driven )

The current highest binning for XP-E2 is Q3, which is 93.9 lum/350mA. The driver I'm using is 1.7amp, which straight math says is 450 lumens at the emitter. If you look at the data sheets -

You can see that lumens x amps is very linear. The reduction in output comes when junction temp (Tj) rises. The forced convection of using a light on a bike is significant - when I use my XML2 driven at 3.5 amps in a solar force host, the light does not even get warm to the touch. According to the data sheets, at Tj=25c, output =100%, and at Tj=50c, output =87%. As I mentioned, I expect the light to be completely cool to the touch while riding, so Tj<50c doesn't seem unreasonable, so therefore 390 emitter lumens doesn't seem unreasonable to expect. Yes, OTF lumens will be less - but then again, I've never seen bike light manufacturers advertise OTF lumen, much less ANSI lumens, so I feel my 390lm estimate is fairly apples-to-apples.

You can see that lumens x amps is very linear. The reduction in output comes when junction temp (Tj) rises. The forced convection of using a light on a bike is significant - when I use my XML2 driven at 3.5 amps in a solar force host, the light does not even get warm to the touch. According to the data sheets, at Tj=25c, output =100%, and at Tj=50c, output =87%. As I mentioned, I expect the light to be completely cool to the touch while riding, so Tj<50c doesn't seem unreasonable, so therefore 390 emitter lumens doesn't seem unreasonable to expect. Yes, OTF lumens will be less - but then again, I've never seen bike light manufacturers advertise OTF lumen, much less ANSI lumens, so I feel my 390lm estimate is fairly apples-to-apples.

Unfortunately the graph only lists output currents to 1000ma. There are other graphs but only one that lists a relationship to current vs. temp. They are using 1000ma as maximum, there is a reason for that. If the emitter gets too hot it can do one of two things; It can simply avalanche and burn out OR it can damage the emitter thus changing it's luminous properties. I've seen the results of the later first hand. It would be interesting if they had included a graph that showed the relationship between Current vs. immediate thermal breakdown.

I routinely measure the heat output from the various bike lamps that I own. Not unusual to measure temperatures on the outside @ 50°C. ( After 10 min. ) and that with a fan blowing on it. I doubt any of those lamps are over-driving the emitters. I think it safe to say that the temperature at the emitter junction is going to be hotter than what is measured on the outside. By how much I can't say. Fifty degrees C. is definitely warm to the touch.

The more current you put through an emitter the faster it will get hot and the hotter it will get thus lowering the output. You can over-drive an emitter in an attempt to get more output but you still have thermal issues that are going to dampen the results. If you have good heat sinking it is going to help but rarely do manufacturers design a lamp with optimal thermal sinking properties.

I've never seen anyone use a 3.5amp driver for a single XM-L or L2 that was intended for bike use. Yeah, I suppose it could be done but it would certainly get damn hot ( real fast ). The L2's can handle heat better but if the lamp runs for a long time I'm certain there would still be an amount of heat related lumen loss. Anything beyond 15% loss is going to be quite noticeable ( not to mention the decrease in run time ) A lot depends of course on the heat sink and how well it can remove heat. Still, everything has limitations and anything that relies on ambient cooling definitely has limitations.

You can see that lumens x amps is very linear. The reduction in output comes when junction temp (Tj) rises. The forced convection of using a light on a bike is significant - when I use my XML2 driven at 3.5 amps in a solar force host, the light does not even get warm to the touch. According to the data sheets, at Tj=25c, output =100%, and at Tj=50c, output =87%. As I mentioned, I expect the light to be completely cool to the touch while riding, so Tj<50c doesn't seem unreasonable, so therefore 390 emitter lumens doesn't seem unreasonable to expect. Yes, OTF lumens will be less - but then again, I've never seen bike light manufacturers advertise OTF lumen, much less ANSI lumens, so I feel my 390lm estimate is fairly apples-to-apples.

No way you're getting 400 lumens out of that xp-e2 red. The lumen per watt isn't a straight line curve, it tapers off, the peak efficiency is the highest between 1 watt and 2 watt per led for the cree xp/xr series. Plus the xp-e series were miniture version of xr-e and were never meant to be driven past 1a.

If you made it yourself, it's unlikely the host has the heatsinking ability to withstand the thermal transfer.

If you bought the solarforce drop in with the cree red (I have the dx/fasttech versions), that's barely a 100 lumens (which is still really really killer bright for a taillight, more than plenty).

Your 390 lumen estimate is wrong.

Quote:

Originally Posted by 01 CAt Man Do

I've never seen anyone use a 3.5amp driver for a single XM-L or L2 that was intended for bike use. Yeah, I suppose it could be done but it would certainly get damn hot ( real fast ). The L2's can handle heat better but if the lamp runs for a long time I'm certain there would still be an amount of heat related lumen loss. Anything beyond 15% loss is going to be quite noticeable ( not to mention the decrease in run time ) A lot depends of course on the heat sink and how well it can remove heat. Still, everything has limitations and anything that relies on ambient cooling definitely has limitations.

Even if you want to do a 3.5amp driver from taskled.com to drive a xm-l/l2, that makes no sense because it's not an efficient use of battery power and lighting. Much better to drive 3 xm-l/l2 at 1amp than one at 3.5 amp.

No way you're getting 400 lumens out of that xp-e2 red. The lumen per watt isn't a straight line curve, it tapers off, the peak efficiency is the highest between 1 watt and 2 watt per led for the cree xp/xr series. Plus the xp-e series were miniture version of xr-e and were never meant to be driven past 1a.

If you made it yourself, it's unlikely the host has the heatsinking ability to withstand the thermal transfer.

If you bought the solarforce drop in with the cree red (I have the dx/fasttech versions), that's barely a 100 lumens (which is still really really killer bright for a taillight, more than plenty).

Your 390 lumen estimate is wrong.

I think he's wrong too but perhaps not as wrong as you may think. Basically we're just arguing about 60 or so lumen. Then again all depends on how the emitter is used. If it is used in a slow flash mode the emitter might not heat up as much. Right now I'm using an over-driven amber XP-E2. I estimate the output around 145 lumen ( @700ma ). That is still damn bright when used in a torch with drop-in style reflectors. Sooner or later I need to do a run test with it to determine how much the output fades when the lamp is left running. I use a strobe-burst-pause mode that is very attention getting.

.... If it is used in a slow flash mode the emitter might not heat up as much...

Low duty cycle keeps average power low. e.g. a 3W light is on only 0.2 seconds every second so duty is 20% and avg power is 0.6W. This only works/helps if the on duration significantly shorter than the time it takes the system to reach thermal equilibrium when on constantly.

I like the look of your mount position, but in this instance it looks like the first good bump is going to knock the light out of that mount. Do you ride any rough trails with this set-up, and has the light fallen out?

Quote:

Originally Posted by ragnar.jensen

I like to mount taillights on the saddle rails.
And there you have it, behind the seatpost and tucked neatly under the saddle

Yes, I have ridden the rough stuff with it. But a bit to my surprise, I have never had it shift position at all.

Ok, I might just have to give it a go then

I suspect the anti-slip tape helps a lot, but that cheapy mount did not give me confidence. That said I have dumped the exact same light onto the trail a few times during my initial attempts to mount it securely, so another bump is not going to make much difference.